Field of the Invention
The present invention relates to a high-voltage power supply apparatus that selectively switches a positive voltage and a negative voltage to be output and an image forming apparatus including the high-voltage power supply apparatus.
Description of the Related Art
A laser beam printer functioning as an image forming apparatus based on an electrophotographic method is provided with a charging apparatus, a developing apparatus, a transfer apparatus, a fixing apparatus, and the like as process members configured to perform image formation. Among the above-described process members, for example, a transfer roller 10 functioning as the transfer apparatus is selectively applied with voltages of both positive and negative polarities (which will be also referred to as a transfer bias). Japanese Patent Laid-Open No. 2006-220976 describes the following configuration. When a recording material 9 exists in a position where an image is transferred to the recording material 9 (which will be also referred to as a transfer nip portion defined by a photosensitive drum 1 on which a toner image is borne and the transfer roller 10), a transfer process is carried out by performing application of a bias having a polarity reverse to that of toner 7. When the recording material 9 does not exist in the transfer nip portion such as a time of a sheet interval, by performing application of a bias having the same polarity as that of the toner 7, the toner 7 adhered on the transfer roller 10 is cleaned, and also a potential fluctuation (which will be also referred to as a memory on the drum) of an image bearing body (the photosensitive drum 1) is avoided.
When a state in the transfer nip portion is switched from a sheet feeding state in which the recording material 9 is present to a non-sheet feeding state in which the recording material 9 is absent, the transfer bias is switched from the bias having the polarity reverse to that of the toner 7 to a bias having′ the same polarity. This switching operation is preferably′ performed in a period during which a margin part of a trailing edge of the recording material 9 (portion between an area where the image is formed and the trailing edge of the sheet) passes through the transfer nip portion. In a case where the switching of the transfer bias is not performed in the period corresponding to this margin part, since the transfer bias is not set at a predetermined potential in the non-sheet feeding state, a problem occurs that a potential of the facing photosensitive drum 1 is affected, and a defective image is generated. The period (time period) during which the trailing edge margin part of the recording material 9 passes through the transfer nip is being shortened due to an increase in a conveyance speed to improve a throughput of a product, and the switching time period of the transfer bias is to be shortened.